The first step in installing Google Checkout is to simply upload the files to your website - if you are sure there has been no changes to your installation.
There are 2 new files that have to be put into your custom template folder: tpl_gc_return_display.php
The files that override existing files are:
So the orders.php in admin is the most important file. If you have changed that file already, either do a file compare or follow these instructions to edit orders.php.